At -15, you can expect this to happen as the difference in temperature and humidity differ so greatly between the outdoors and inside your home. This article suggests decreasing the humidity inside your home to shift the dew point and prevent ice and condensation formation on the inside of your windows at these extreme temperatures: http://www.improvinghome.com/content/why-there-water-and-ice-my-windows

At these temperatures, it's normal for the air to be a bit colder around the windows as glass does not provide as resilient insulation as solid walls.

I agree with JCaption - to provide a complete answer we will need more details. However, if you don't have these issues above -15 degrees, it's likely simply due to the rare, deep freeze.